Certain power plants, such as those for generating electricity, use pulverized coal as a fuel. Typically, the pulverized coal is entrained in a temperature-controlled hot air stream to the combustor where the coal is burned to create hot gases that either drive a turbine generator directly or heat water to provide steam for driving a turbine-generator. The temperature of the hot air stream into which the pulverized coal is introduced is important, and one way of controlling the temperature is by tempering a relatively hotter air stream with a relatively colder tempering air stream. Dampers-are commonly employed as a means for adjusting the relative proportions of the relatively hotter and the relatively colder air streams for the purpose of attaining a desired average temperature for the tempered hot air stream into which the pulverized coal is to be introduced.
Even with such control, certain pre-existing coal-fired power plant installations have experienced problems, such as pre-ignition of the pulverized coal. Investigation of certain of these installations has now revealed that substantial temperature gradients can exist across the expanse of the tempered hot air stream into which the pulverized coal is introduced; and hence, it is speculated that pre-ignition of the pulverized coal is due to the presence of certain hot spots in the tempered hot air stream. While a gross introduction of relatively colder tempering air might avoid premature ignition of the pulverized coal, such a solution would inherently detract from process efficiency. Accordingly, it has now been realized that management of the air tempering process by only using the existing dampers to adjust the relative proportions of relatively hotter and relatively colder air streams that form the tempered hot air stream is apt to be a non-optimal solution, and possibly even an unsatisfactory one.
The present invention has provided a solution for this temperature gradient problem in an existing pulverized coal-fired power plant installation. More specifically, the invention has attained significant reductions in temperature gradient across the expanse of the tempered hot air stream in a way that provides temperatures conducive to optimizing process efficiency without creating hot spots that may cause pre-ignition of the fuel. Moreover, the air mixer device that accomplishes this result is a static structure that requires no moving parts and can be fit to both new and existing power plant installations.
Speaking generally and briefly by way of introduction with regard to the installation of the mixer device in a coal-fired power plant, one may define the present invention as a static air mixer device that comprises respective first and second inlets, or entrances, into which the relatively hotter and the relatively colder air streams are respectively introduced, channel means for sub-dividing each entrance stream into a number of sub-divided streams that pass through respective channels which are alternately side-by-side with respective channels through which the sub-divided streams of the other entrance stream pass, and respective outlets, or exits, from whence the side-by-side streams combine to form the tempered hot air stream across the expanse of which the temperature gradient is significantly reduced from that which was found to exist in the aforementioned power plant installations lacking such an air mixer device. The interleaved sub-divided streams accelerate the mixing process, resulting in the attainment of greater homogeneity within a shorter distance from the confluence of the relatively hotter and the relatively colder air streams, than was the case before the incorporation of the air mixer device. Moreover, because the device of the present invention can be fabricated by means of conventional fabrication techniques in an economical manner, it is well-suited for convenient and expeditious installation not only in pre-existing installations, but in new ones as well. With sufficiently strong construction and lacking moving parts, it can provide long, useful, maintenance-free service.
The coal-fired power plant described herein in detail represents one inventive example, and it is contemplated that principles of the invention will also enjoy other usage in other inventive examples. Hence, in its most comprehensive aspect, the present invention relates to a device for creating a mixture stream from two separate fluid streams where the objective of the mixer device is to create improved homogeneity in a particular characteristic of the resulting mixture, be such characteristic the temperature gradient across the resulting mixture, or some other characteristic or quality. Indeed, the constituents of each entering stream themselves do not even have to be homogeneous; for example, air itself is a mixture of various constituents, and it may even include certain entrained material like dust, pollen, or other particulates.
In one specific exemplary aspect of the invention as an air mixer device for a pulverized coal-fired power plant, an exemplary air mixer device creates a tempered hot air stream from a stream of relatively hotter air and a stream of relatively colder air. Pulverized coal is entrained with this tempered hot air stream to form a two-phase mixture stream that flows to a combustor where the coal is combined with other combustion air and burned to provide the thermal energy input to the power plant. The air mixer device creates the tempered hot air stream from the separate relatively hotter and relatively colder air streams that enter the device at respective inlets. Each entering stream is divided into a number of separate, parallel, side-by-side, spaced apart sub-divided streams, and as they pass through the device, the relatively hotter sub-divided streams and the relatively colder sub-divided streams are arranged alternately side-by-side so that as the two sets of sub-divided streams exit the device, the relatively hotter and relatively colder sub-divided streams are side-by-side. As these interleaved sub-divided streams mix upon exiting the device, they create a tempered hot air stream across the expanse of which the temperature gradient is significantly less than it was in the same installation without the mixer device. Hence, the mixer device has been shown to promote more uniform temperature over the expanse of the tempered hot air stream than in a pre-existing power plant that did not include the air mixer device, but instead relied merely on the use of dampers in the respective ducts conveying the respective relatively hotter and relatively colder air streams to the mixing zone. Other aspects of the fluid mixing device of the present invention extend beyond this one specific aspect of a pulverized coal-fired power plant.
Further features, advantages, and benefits of the invention, along with those already expressly mentioned, will be seen in the ensuing description and claims, which should be considered in conjunction with the accompanying drawings of a presently preferred embodiment of the invention, representing the best mode contemplated at this time in carrying out the invention.